1. Field of the Invention
The present invention relates to an ink for inkjet recording. More specifically, it relates to an ink for inkjet recording which is less prone to bleed and spread on ordinary paper even by high speed recording and capable of realizing clear images.
2. Description of Related Art
With the progress of techniques for manufacturing a head provided with nozzles of high integration and high definition in recent inkjet recording technology, it is becoming possible to discharge finer ink droplets to print images having high resolution of 1200 dpi or more at high speed. In particular, high quality images can be recorded at high resolution on a medium exclusive to inkjet printing provided with an ink receptor layer on its surface.
However, even by the recent inkjet recording technology, images recorded on ordinary paper are still prone to be poor in recording quality. Spreading and bleeding (mixing of colors) of the ink at the boundary of ink dots adjacent to each other on the ordinary paper are pronounced and problematic, which have not been solved yet.
Japanese Unexamined Patent Publication No. HEI 4 (1992)-259566 proposes a technique of preventing the ink from spreading and bleeding. According to this technique, scanning is performed several times to discharge ink droplets from all discharge outlets in such a manner that ink dots adjacent to each other are recorded on a recording material at an increased time interval. However, this recording technique is problematic in that the recording speed becomes very low and the spreading and bleeding of the ink cannot be avoided depending on the combination of the ink and the recording material.
Japanese Unexamined Patent Publication No. HEI 6(1994)-92008 proposes ink containing at least acetylene glycol and showing a contact angle of 40′ or smaller 5 seconds after dropping onto sized paper. However, the contact angle obtained 5 seconds after dropping is meaningless in view of phenomena shown by the ink droplets in the actual inkjet recording, and therefore it seems irrelevant to the prevention of the spreading and bleeding of the ink.
Japanese Unexamined Patent Publications Nos. HEI 6(1994)-136307 and HEI 6(1994)-136308 propose to apply ink whose advancing tension and an advancing contact angle are numerically controlled to the inkjet recording. However, it is not sure how the advancing tension and the advancing contact angle of the ink are related to the permeation speed of the ink and the smearing of the images. There is uncertainty as to whether or not this ink is capable of realizing high quality images by high speed printing.
Japanese Unexamined Patent Publication No. HEI 10(1998)-330666 proposes ink showing wet time of 0.2 sec or less according to Bristow method and the advancing contact angle of 40 to 70° with respect to a recording material of 10 sec or more. The wet time according to Bristow method is a value for indicating roughness of the paper, which interprets the ink permeation from a macroscopic viewpoint. Therefore, it is quite different from the permeation of the superfine ink droplets in the field of the current inkjet recording and thus irrelevant to the spreading of the ink and the high speed printing.
Japanese Unexamined Patent Publication No. 2000-144028 discloses pigment ink wherein the rate of change of a dynamic contact angle over time on ordinary paper is 0.5 to 3.5°/s. However, this ink permeates very slowly, which inevitably causes the bleeding.
Japanese Unexamined Patent Publication No. SHO 62 (1987)-30169 discloses ink containing lower alkylene glycol monoacrylether as means of improving the fixing property of the ink. Further, Japanese Unexamined Patent Publication No. SHO 55 (1980)-80477 discloses a technique of preventing the ink from spreading on a paper surface in a lateral direction by using carboxymethyl cellulose. These techniques do not create big problems in fixing property and printing quality. However, both of the techniques are intended to enhance affinity between ink and paper to improve permeability, thereby preventing uneven permeation. Therefore, the ink permeates not only on the paper surface but also deeply in a depth direction of the paper. This is disadvantageous because the image density decreases.
On the other hand, Japanese Unexamined Patent Publication No. SHO 55 (1980)-42858 discloses a technique of using a surfactant having specific surface tension to decrease the spreading and blurring of the ink which may be caused upon overlaying other colors thereon. However, the ink containing the surfactant permeates deep into the paper after printing, which also decreases the image density.
Japanese Unexamined Patent Publication No. HEI 4 (1992)-170478 proposes ink whose initial contact angle and surface tension are large to prevent uneven permeation of the ink on the ordinary paper. However, in order to enhance the image density using this ink, the ink needs to be used in a large amount. This deteriorates the fixing property and the anti-bleeding property of the ink.